A paramyxovirus is one of many viruses in the family Paramyxoviridae. This family is made up of large viruses with a single strand of ribonucleic acid (RNA) and an envelope. The family contains many human and animal pathogens. Paramyxovirus examples include measles, mumps, respiratory syncytial virus and canine distemper.

The parameters for inclusion in the paramyxovirus group include the size of the virus, the presence of an envelope and a single-stranded, negative-sense RNA genome. Paramyxoviruses vary in size from 150 nanometers to 200 nanometers in diameter. Their envelope is made up of molecules containing both fat and proteins known as lipoproteins. The nucleocapsid of the viruses, which is the internal structure holding the genetic material, is helical in shape and has a protein shell.

A paramyxovirus replicates by fusing with a host cell membrane and using the cell's machinery to reproduce. Paramyxoviruses, unlike other enveloped viruses, have two molecules — glycoproteins — they use to attach to the cell membrane and enter the cell. The paramyxovirus genetic sequence is made of antisense RNA. This RNA is a backward code for proteins, so the paramyxovirus uses its own RNA polymerase enzyme to transcribe the RNA into positive sense, or RNA that isn't backward. The positive-sense RNA can then be used to produce proteins that help a virus particle to replicate correctly.

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The family paramyxoviridae is divided into two subfamilies. These are the subfamily paramyxovirinae and the subfamily pneumovirinae. The subfamilies are divided according to genome organization, morphological criteria and protein characteristics and activity. The pneumovirinae subfamily contains the respiratory syncytial virus, which is the most common cause of lower respiratory tract infections worldwide. The virus also causes pneumonia in infants.

The measles virus is a member of the genus morbillivirus, which is part of the paramyxovirinae subfamily. Measles is an infection of humans that is spread through inhalation of particles with the disease, causing fever, rash, cough and a runny nose. Complications such as ear infections, pneumonia or encephalitis can occur, and measles can occasionally be fatal. The paramyxovirinae subfamily also contains the mumps virus and parainfluenza viruses.

Paramyxoviruses were originally classified as belonging to the family orthomyxoviridae. This was the result of the similarities in glycoprotein activity in both groups. Orthomyxoviridae have different protein expression, genome organization and replication strategies, so the families were separated, with the paramyxovirus name retaining some of the historical association between the two groups.

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